Chapter 20 IPSec VPN
Usually, you should select ESP. AH does not support encryption, and ESP is more suitable with NAT.
20.1.1.3 Encapsulation
There are two ways to encapsulate packets. Usually, you should use tunnel mode because it is more secure. Transport mode is only used when the IPSec SA is used for communication between the ZyWALL and remote IPSec router (for example, for remote management), not between computers on the local and remote networks.
"The ZyWALL and remote IPSec router must use the same encapsulation.
These modes are illustrated below.
Figure 196 VPN: Transport and Tunnel Mode Encapsulation
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Original Packet | IP Header | TCP | Data |
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Transport Mode Packet |
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IP Header | AH/ESP | TCP | Data |
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Tunnel Mode Packet |
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IP Header | AH/ESP | IP Header | TCP | Data |
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In tunnel mode, the ZyWALL uses the active protocol to encapsulate the entire IP packet. As a result, there are two IP headers:
•Outside header: The outside IP header contains the IP address of the ZyWALL or remote IPSec router, whichever is the destination.
•Inside header: The inside IP header contains the IP address of the computer behind the ZyWALL or remote IPSec router. The header for the active protocol (AH or ESP) appears between the IP headers.
In transport mode, the encapsulation depends on the active protocol. With AH, the ZyWALL includes part of the original IP header when it encapsulates the packet. With ESP, however, the ZyWALL does not include the IP header when it encapsulates the packet, so it is not possible to verify the integrity of the source IP address.
20.1.1.4 IPSec SA Proposal and Perfect Forward Secrecy
An IPSec SA proposal is similar to an IKE SA proposal (see Section 20.4.1.2 on page 307), except that you also have the choice whether or not the ZyWALL and remote IPSec router perform a new DH key exchange every time an IPSec SA is established. This is called Perfect Forward Secrecy (PFS).
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